Nervous System Overview and Functions

Nervous Tissue Overview

• The nervous system is complex and fundamental to conscious experience, personality, and behavior
• It coordinates with the endocrine system
• Endocrine system communicates via chemical messengers (hormones) released into the bloodstream
• Nervous system uses electrical and chemical signals to transmit messages between cells

Nervous System Tasks

• Sense organs gather and transmit information to the spinal cord and brain
• The brain and spinal cord process this information, recalling past experiences to determine the appropriate response
• The brain and spinal cord command muscles and glands to execute the response

Nervous System Subdivisions

• Central Nervous System (CNS):
  • Brain and spinal cord
  • Enclosed in bony coverings (cranium and vertebral column)
• Peripheral Nervous System (PNS):
  • All nervous system components outside the brain and spinal cord
  • Contains:
    • Nerves: Bundles of nerve fibers (axons) with connective tissue
    • Ganglia: Knot-like swellings in nerves containing neuron cell bodies

Peripheral Nervous System Subdivisions (Functional)

• Sensory (Afferent) Division:
  • Carries sensory signals from various receptors to the CNS
  • Includes:
    • Somatic Sensory Division: Skin, muscles, bones, and joints
    • Visceral Sensory Division: Internal organs (heart, lungs, stomach, bladder)
• Motor (Efferent) Division: Carries signals from CNS to effectors that carry out the response
  • Includes:
    • Somatic Motor Division: Skeletal muscles (voluntary movements and reflexes)
    • Visceral Motor Division (Autonomic Nervous System):
      • Sympathetic Division: Prepares the body for action (accelerates heart rate, respiration)
      • Parasympathetic Division: Calming effect (slows heart rate, stimulates digestion and urination)

Universal Neuron Properties

• Excitability (Irritability) : Responds to stimuli
• Conductivity: Carries electrical signals
• Secretion: Releases chemical neurotransmitters to stimulate next cells

Neuron Functional Classes

• Sensory (Afferent) neurons: Detect stimuli and transmit signals to the CNS
• Interneurons (Association neurons): Located entirely within the CNS, process, store, and retrieve information, and make decisions about how the body should respond to stimuli
• Motor (Efferent) neurons: Transmit signals from the CNS to muscles and glands (effectors)

Neuron Structure

• Soma (Neurosoma, Cell Body): Control center, contains nucleus and other organelles
• Dendrites: Receive signals from other neurons
• Axon (Nerve Fiber): Carries signals away from the soma
• Axon Collaterals: Branches of axons
• Synaptic Knobs (Terminal Buttons): Form synapses with the next cell and release neurotransmitters
• Myelin Sheath: Insulating layer around some axons for faster signal transmission
  • Formed by oligodendrocytes in CNS and Schwann cells in PNS
  • Consists of 20% protein and 80% lipid
  • Nodes of Ranvier and internodes are segmented sections

Neuron Classification

• Multipolar: Most common; many dendrites, one axon (brain and spinal cord)
• Bipolar: One axon, one dendrite (olfactory cells, retina, inner ear)
• Unipolar: One process leading away from the soma (sensory neurons); from skin/organs to spinal cord
• Anaxonic: Many dendrites, no axon (in the brain)

Support Cells (Neuroglia)

• Outnumber neurons in the nervous system.
• Provide structural support and protection
• Types include:
  • Oligodendrocytes: Form myelin sheaths in CNS
  • Ependymal Cells: Line brain cavities, secrete and circulate cerebrospinal fluid (CSF)
  • Microglia: Small macrophages that remove cellular debris
  • Astrocytes: Most abundant, form framework, stimulate blood-brain barrier formation, convert glucose to lactate, and provide nourishment to neurons
  • Schwann Cells: Form myelin sheaths in PNS; assist in fiber regeneration
  • Satellite Cells: Surround somas in ganglia, regulate chemical environment

Myelin

• Insulating layer around some axons
• Speeds signal transmission

Diseases of the Myelin Sheath

• Multiple Sclerosis: Degradation of myelin in the CNS

Glial Cells and Brain Tumors

• Tumors: Masses of rapidly dividing cells
• Brain Tumors: Arise from meninges, metastasis from other organs, or glial cells
• Gliomas: Rapidly growing, highly malignant brain tumors